The present invention relates generally to optical interferometry. More particularly, the present invention relates to a method and apparatus for determining the location of an imager plane with respect to a camera-mounting plane of an imager assembly.
In a conventional digital camera, an image beam is directed through a lens and onto an imager or image sensor comprised of an array of sensing elements, for example a Charge Coupled Device (CCD). In order to provide a focused image, the lens and the imager need to be properly positioned, relative to each other, within the digital camera.
The steps in a CCD based imager manufacturing process are as follows. Multiple CCD imager arrays are processed together on a single silicon wafer. Imager dies, composed of a single CCD imager array, are diced from the wafer and positioned and glued into specially designed packages. A flat transparent plate called the imager cover glass is then glued into the specially designed package at a location that is offset from the imager die to hermetically seal the specially designed package. This hermetically sealed package containing the imager die is then mounted into a camera-mounting plate that includes a reference plane to facilitate proper mounting into the camera. The camera itself will include a camera reference plane to receive the camera mounting plate from the imager package. In a film camera, film rails usually define the camera reference plane. Optionally, the package can include the camera-mounting plate and reference plane, which would eliminate this last step of mounting the hermetically sealed package into a camera mounting plate.
In order to ensure that the CCD is positioned properly in the camera, the location of the CCD needs to be determined. Such a location can be determined relative to a reference surface or reference plane.
A Coordinate Measuring Machine (CMM) is an example of an apparatus employed to determine the location of an object relative to a reference plane. Conventionally, the object is retained in a suitable holder on an optical bench. In one method to determine the location of an object, three points on a reference plane, approximately 120 degrees apart, are measured to define the reference plane; the coordinates of the three points are tracked in the x, y and z directions. A point on the object is then measured relative to the reference plane, and the distance from the reference plane is calculated. Conventional CMMs have contact probes for intimately contacting each of the points defining the reference plane and the object, such as those described in U.S. Pat. No. 5,428,446 issued Jun. 27, 1995 to Ziegart et al. entitled Measurement Instrument with Interferometer and Method, U.S. Pat. No. 5,446,545 issued Aug. 29, 1995 to Taylor entitled Method of and Apparatus for Calibrating Machines Including a Measuring Probe and a Measuring Apparatus, and U.S. Pat. No. 4,929,082 issued May 29, 1990 to Webber entitled Laser Linear Distance Measurement System and Apparatus. These references include interferometers that monitor the displacement of the machine axes. In contrast, non-contacting methods, such as optical triangulation, are described in U.S. Pat. No. 4,373,804 issued Feb. 15, 1983 to Pryor et al entitled Method and Apparatus for Electro-Optically Determining the Dimension, Location and Attitude of Objects, and U.S. Pat. No. 5,510,625 issued Apr. 23, 1996 to Pryor et al. entitled Method and Apparatus for Electro Optically Determining the Dimension, Location and Attitude of Objects. 
Another technology known as low-coherence light interferometry has also been used to measure physical properties of an object. U.S. Pat. No. 5,659,392 issued Aug. 19, 1997 to Marcus et al. entitled Associated Dual Interferometric Measurement Apparatus for Determining a Physical Property of an Object, and U.S. Pat. No. 5,596,409 issued Jan. 21, 1997 to Marcus et al. entitled Associated Dual Interferometric Measurement Method for Determining a Physical Property of an Object, disclose an associated dual interferometric apparatus and method for measuring physical properties of an object, such as thickness, group index of refraction, and distance to a surface. U.S. Pat. No. 5,757,485 issued May 26, 1998 to Marcus et al. entitled Digital Camera Image Sensor Positioning Method Including a Non-Coherent Interferometer, and U.S. Pat. No. 5,757,486 issued May 26, 1998 to Marcus et al. entitled Digital Camera Image Sensor Positioning Apparatus Including a Non-Coherent Light Interferometer, disclose a digital camera image sensor positioning apparatus and method which includes a low-coherence light interferometer. The apparatus and method include a removable optical probe assembly mounted to a digital camera. The low-coherence light interferometer is in communication with the optical probe. assembly to determine a depth of an image sensor residing within a digital camera, relative to a reference surface. U.S. Pat. No. 6,075,601 issued Jun. 13, 2000 to Marcus et al. entitled Optical Probe Calibration Apparatus and Method describes an optical probe calibration apparatus used for calibrating the optical probes used in U.S. Pat. Nos. 5,757,485 and 5,757,486 referenced above. These three to the camera reference surface.
Heretofore, a skilled operator was required to install the imager in the camera and subsequently assemble the camera before finding out if the imager was properly focused. Several steps were required, including securing the imager with 3 or 4 screws onto the camera-mounting plane, and inserting a measurement optical probe into the camera body and locking the probe into the lens flange-mounting ring before a measurement could be initiated. Before mounting the measurement optical probe into the camera body, the camera electronics needed to be turned on and the electronic shutter needed to be opened. Full camera assembly and substantial skilled operator intervention were required before an assessment of imager focus could be made. If the imager was out of focus, the camera had to be disassembled and the imager replaced. In order to calibrate the measurement optical probe, an external calibration fixture was also required. The distance from the camera-mounting ring to the reference surface in the external calibration fixture is better suited for measurement with an external technique, such as provided by a CMM machine.
While internal apparatus and methods may have achieved a certain level of success, the internal apparatus is not readily transportable nor simple to use. Further, the methods are time consuming and quite often are dependent on the skill of the operator.
Accordingly, a need continues to exist for an apparatus and method for determining the position of an image sensor in a digital camera. Furthermore, there is a need to properly predict the position of an image sensor before permanently physically mounting the image sensor inside the digital camera. The apparatus needs to be robust, transportable and simple to use. The method must be fast, provide objective results independent of the operator, and provide accurate and consistent results.
The need is met according to the present invention by providing a method for determining whether an imager assembly outside of a camera body meets predetermined focus specifications, wherein the imager assembly includes an image sensor and a camera mounting plate having reference features adapted to cooperate with alignment features in the camera body to locate the image sensor at a predetermined focal plane, including the steps of: mounting the imager assembly onto an imager mounting apparatus having equivalent alignment features, and utilizing low-coherence light interferometry to determine whether the image sensor will meet predetermined focus specifications when mounted in a camera body.
The present invention also provides an imager mounting apparatus to receive an imager assembly in a predetermined orientation for determining whether an imager assembly outside of a camera body meets predetermined focus specifications, including: an optical probe with a pellicle reference surface; a camera body mounting equivalent with equivalent alignment features for receiving and aligning the imager assembly in a predefined orientation; and a plurality of clamps to lock in the predetermined orientation.
The present invention also provides an interferometric-based measurement system for determining whether an imager assembly outside of a camera body meets predetermined focus specifications, including: a low coherence light interferometer; an imager mounting apparatus including an optical probe having an optical probe chuck; an optical fiber cable for coupling light from the interferometer to the optical probe chuck; and a computer for processing data collected by the interferometer, wherein the data is used to determine whether the imager assembly meets predetermined focus specifications.
The present invention also provides a method for calibrating an absolute distance to a reference surface for determining the position of an imager plane relative to an image sensor camera-mounting reference plane in an imager assembly, including the steps of: mounting a flat reference plate onto an imager mounting reference surface; and utilizing low coherence light interferometry to determine the distance between the imager mounting reference surface and a pellicle reference surface (known as PPxe2x80x2) of the imager mounting apparatus.
The present invention also provides a method for determining a position of an imager plane relative to an image sensor camera-mounting plane in an imager assembly, including the steps of: temporarily mounting the imager assembly onto an imager mounting apparatus having an imager mounting reference surface such that the imager sensor camera-mounting reference plane and the imager mounting reference surface are in intimate contact; wherein the imager mounting apparatus includes an optical probe with a pellicle reference surface in a predetermined orientation with respect to the imager mounting reference surface such that the pellicle reference surface is disposed at a first depth relative to the imager mounting reference surface; utilizing low-coherence light interferometry to determine: (i) a second depth from the pellicle reference surface to a front surface of the optically transparent plate, (ii) an optical thickness of the imager cover glass, and (iii) a third depth from a back surface of the imager cover glass to the imager plane; and calculating the optical position of the imager plane relative to the imager sensor camera-mounting reference plane.